NO. 3539 HAMMERHEAD SHARKS GILBERT 9 



fish classification (Gill, 1861, pp. 403-412; Jordan, 1923; Berg, 1940), 

 while others (Regan, 1906, pp. 722-758; Norman, iinpubl. ms.) have 

 regarded this group as a subfamily of the Carcharhinidae. Berg's 

 action apparently was based primarily on the studies by White (1937), 



Although White (1937) retained the Sphyrnidae as a distinct family, 

 she gave no convincing reasons supporting this decision. Little was 

 said about the relationships of the sphyrnids and carcharhinids, and 

 the only places where they were compared were in the key and in the 

 discussion of the pelvic claspers. While the pelvic claspers obviously 

 constitute a valuable taxonomic character for the separation of various 

 species, their value in determining interspecific relationships has been 

 subject to question. Accordmg to White (1937, p. 94), "They [the 

 claspers] tend to vary within the order [Galea] ... so inconsistently 

 that they are of little value in establishing relationships." Final 

 judgment should be reserved until a more intensive study is made of 

 most or all the species. 



The sphyrnids and carcharhinids share a large number of mor- 

 phological characters. Both have (1) the first dorsal fin anterior to 

 the pelvic fins; (2) no naso-oral groove; (3) the keels of the dermal 

 denticles complete and dentate on the posterior edge and situated on a 

 flat surface; (4) a nictitating membrane; (5) teeth few in number, 

 never multicuspid ate; (6) an asterospondylic type of vertebrae, which 

 has the appearance of a "maltese cross" when viewed in cross section; 

 (7) a "scroll type" spiral valve; (8) three rows of valves on the conus 

 arteriosus; and (9) an ovo viviparous type of reproduction. 



The fact that the sphja-nids and carcharhinids have so many basic 

 characters in common suggests a close relationship; however, most 

 of these featm-es are foimd also in other famihes, some of which are 

 not intimately related to these two groups. The "maltese cross" 

 pattern of the vertebrae is as well developed in the family Triakidae 

 as it is in the carcharhinids and sphyrnids. In addition, the nicti- 

 tating membrane, which has been thought to be unique to the latter 

 two families, also occurs in the triakids (V. G. Springer and J. A. F. 

 Garrick, in Htt.). 



Recent unpublished studies by Dr. Shelton P. Applegate indicate 

 that the hammerheads, in addition to the pronounced lateral expansion 

 of the head, differ from carcharhinid sharks in the following ways: 

 (1) The vertebrae, in cross section, show a convexity of the uncalcified 

 canal of the neural and haemal arches that is "butterfly-like" in shape, 

 rather than straight, and (2) in the teeth the lower blade has a smooth, 

 symmetrical arc, whereas in carcharhinid genera there is a definite 

 break in the arc on the side away from the crown. I regard these 

 differences as sufficient justification for continued recognition of the 

 hammerheads as a distinct family. 



